guide for me for new and rehabilitated pavement structures

8/3/2019 Guide for ME for New and Rehabilitated Pavement Structures

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SS-i

Copy No.

Guide for Mechanistic-Empirical DesignOF NEW AND REHABILITATED PAVEMENTSTRUCTURES

FINAL DOCUMENT

APPENDIX SS:HYDRAULIC DESIGN, MAINTENANCE, AND

CONSTRUCTION DETAILS OF SUBSURFACE DRAINAGE

SYSTEMS

NCHRP

Prepared forNational Cooperative Highway Research Program

Transportation Research Board

National Research Council

Submitted by

ARA, Inc., ERES Division

505 West University Avenue

Champaign, Illinois 61820

February 2001


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SS-ii

Acknowledgment of Sponsorship

This work was sponsored by the American Association of State Highway and Transportation

Officials (AASHTO) in cooperation with the Federal Highway Administration and wasconducted in the National Cooperative Highway Research Program which is administered by the

Transportation Research Board of the National Research Council.

Disclaimer

This is the final draft as submitted by the research agency. The opinions and conclusions

expressed or implied in this report are those of the research agency. They are not necessarilythose of the Transportation Research Board, the National Research Council, the Federal

Highway Administration, AASHTO, or the individual States participating in the National

Cooperative Highway Research program.

Acknowledgements

The research team for NCHRP Project 1-37A: Development of the 2002 Guide for the Design of

New and Rehabilitated Pavement Structures consisted of Applied Research Associates, Inc.,

ERES Consultants Division (ARA-ERES) as the prime contractor with Arizona State University

(ASU) as the primary subcontractor. Fugro-BRE, Inc., the University of Maryland, andAdvanced Asphalt Technologies, LLC served as subcontractors to either ARA-ERES or ASU

along with several independent consultants.

Research into the subject area covered in this Appendix was conducted at ARA-ERES. The

authors of this Appendix are Mr. Jagannath Mallela, Leslie Titus-Glover, and Dr. Michael I.Darter.

Foreword

This appendix is a supporting reference to the subdrainage guidance presented in PART 3,

Chapter 1 of the Design Guide. Some sections of the referenced chapter are repeated here foremphasis and continuity. Of particular interest are sections on hydraulic design of the drainage

componentspermeable bases, separator layers, edgedrains, and outletsdrainage construction,

and maintenance.


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APPENDIX SSHYDRAULIC DESIGN, MAINTENANCE, AND CONSTRUCTION

DETAILS OF SUBSURFACE DRAINAGE SYSTEMS

Introduction

As early as 1820, John McAdam noted that, regardless of the thickness of the structure, manyroads in Great Britain deteriorated rapidly when the subgrade was saturated (1). It isrecognized today that excess moisture in pavement layers, when combined with heavy trucktraffic and moisture-susceptible materials, can reduce service life. Below-freezing temperaturescan contribute to durability problems of saturated materials. Failures related to moisture-accelerated damage continue to take place to this day. In recognition of the impact moisturecan have on pavement performance, the AASHTO Design Guide incorporated an empiricaldrainage coefficient into the 1986 design equations. This coefficient increased awareness andencouraged design of pavements with permeable drainage layers.

The M-E procedures consider the effects of excess moisture on unbound granular andsubgrade layers through reductions in layer moduli. The incremental damage accumulationapproach makes it possible to consider seasonal changes in unbound layers directly. Thisapproach also makes it possible to consider changes in layer properties over time. Theseinclude the erosion of layers with subsequent loss of support conditions for PCC pavements andincreased infiltration of moisture into AC pavements from cracking over time. However, thepractical considerations of subsurface drainage design are important because not all of theseeffects can be considered in the M-E design procedure.

Sources of Moisture in Pavements

It is important to identify the sources of moisture in pavements before devising ways to combatit. Moisture in the subgrade and the pavement structure can come from many different sources(see figure 1). Water may seep upward from a high groundwater table due to capillary suctionor vapor movements (2), or it may flow laterally from the pavement edges and side ditches.Another important source of water in pavements is surface infiltration of rainwater through joints,cracks, shoulder edges, and various other defects, especially in older deteriorated pavements.A study by the Minnesota Department of Transportation indicated that 40 percent of rainfallenters the pavement structure (3). In fact, Demonstration Project 87, Drainable PavementSystems, states that surface infiltration is the single largest source of moisture-related problemsin PCC pavements (4). Although AC pavements do not contain joints, they develop cracks,longitudinal cold joints that crack, and pavement edges that provide ample opportunity for waterto infiltrate the pavement structure and cause damage.

This chapter addresses moisture infiltrating the pavement structure through the surface.Groundwater seepage is usually considered a geotechnical problem which needs to beaddressed during embankment design and will not be discussed in this Guide.


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Table 1. Moisture-related distresses in PCC pavements (adapted from 8).

TypeDistress

ManifestationMoistureProblem

ClimaticProblem

MaterialProblem

Load-Associated?

P

Spalling Possible Freeze-thaw Cycles Mortar No Y

ScalingYes Freeze-thaw Cycles

ChemicalInfluence

No YFin

D-Cracking Yes Freeze-thaw CyclesAggregateExpansion No Y

SurfaceDefects

Crazing No No Rich Mortar No Y

Weak

Blow-up No Temperature ThermalProperties

No Y

Pumping andErosion

Yes Moisture InadequateStrength

Yes

Faulting Yes Moisture-Suction

Erosion-Settlement

Yes

SurfaceDeformation

Curling/Warping Yes Moisture &

Temperature

Moisture andTemperatureDifferentials

No Y

Corner Yes Moisture Cracking followsErosion

Yes

DiagonalTransverseLongitudinal

Yes Moisture Follows Erosion Yes

Cracking

Punchout (CRCP) Yes Moisture High deflectionsfollow erosion Yes

S

S-3


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Table 2. Moisture-related distresses in conventional AC pavements (adapted from

TypeDistress

ManifestationMoistureProblem

ClimaticProblem

MaterialProblem

Load-Associated?

A

Bump orDistortion

ExcessMoisture

Frost Heave VolumeIncrease

No

Corrugationor Rippling

Slight Moisture andTemperature

Unstable Mix Yes Y

Stripping Yes Moisture Loss of Bond No Y

RuttingExcess in

Granular Layers orSubgrade

MoisturePlastic

Deformation,Stripping

Yes Y

Depression Excess Moisture Suction &Materials

Settlement, FillMaterial

No

SurfaceDeformation

Potholes Excess Moisture Moisture,

TemperatureStrength-Moisture

Yes Y

Longitudinal not-in-wheel-path andlongitudinal in-

wheel-path

No (acceleratescrack severity)

No Construction No FaCons

Alligator (fatigue) Yes (acceleratescrack severity)

Spring-ThawStrength Loss

Thickness Yes Yes

Transverse No (acceleratescrack severity)

Low Temp,F-T Cycles

ThermalProperties

No Yes,Susc

Cracking

Slippage Yes No Loss of Bond Yes Yes

S

S-4


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Approaches to Address Moisture in Pavements

A major objective in pavement design should be to keep the base, subbase, subgrade, andother susceptible paving materials from becoming saturated or even being exposed to constanthigh moisture levels over time. Four approaches commonly employed to control or reducemoisture problems are listed below:

Prevent moisture from entering the pavement system. Use materials that are insensitive to the effects of moisture. Incorporate design features to minimize moisture damage. Quickly remove moisture that enters the pavement system.

It is important to recognize that no single approach can completely negate the effects ofmoisture on the pavement system under heavy traffic loading over many years. Thus, it is oftennecessary to employ all approaches in combination, particularly for heavy traffic loadingconditions.

Basic Subsurface Drainage TerminologyThis section introduces some of the subdrainage components referred to throughout thischapter, along with short discussions of their functions and salient characteristics. Detaileddesign considerations are addressed later in this chapter.

Permeable Base: An open-graded drainage layer with a minimum laboratory permeabilityvalue of 1000 ft/day. This layer could be asphalt-treated, cement-treated,or untreated, depending on structural requirements. The primary functionof this layer is to collect water infiltrating the pavement and to move it tothe edgedrains within an acceptable timeframe. The aggregate used forthe permeable base should be crushed (with at least two mechanically

fractured faces) and wear resistant. Asphalt-treated drainage layers mustbe treated with a stiff AC binder to prevent draindown. Additionalspecifications on treated open-graded materials can be found in PART I,Chapter 2 of the Guide. Guide specifications are also available throughthe FHWA.

Separator Layer: An impermeable layer of aggregate material (treated or untreated) orgeotextile or a combination thereof placed between the permeable baseand the subgrade. The separator layer has three main functions: (a) tomaintain separation between permeable base and subgrade and preventthem from intermixing, (b) to form an impermeable barrier that deflectswater from the permeable base horizontally toward the pavement edge,

and (c) to support construction traffic.

Edgedrains: Longitudinal pipes that run along the pavement length. They are placed 2inches from the bottom of a trench dug on the side of the pavementadjacent to the laneshoulder joint. They collect water discharged fromthe pavement structure and transfer it to the outlets. Pipe edgedrains andprefabricated geocomposite edgedrains (PGEDs) are the two types ofcommonly available edgedrains. Smooth-walled pipes having adequatestrength to withstand loads placed on them are recommended for use aspipe edgedrains. Pipes should conform to the appropriate State orAASHTO specification.


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Outlets: Short pipes that carry the water from the edgedrains to the side ditches.Non-perforated smooth, rigid pipes are recommended for outlets. Thispipe must resist construction and maintenance traffic.

Headwall: Headwalls made of PCC are used to house drainage outlets to preventthem from damage caused by routine maintenance activities. They also

help prevent slope erosion and aid in locating outlet pipes. Headwallsshould be placed flush with the slope of the embankment so that routinemaintenance activities are not impaired. Removable rodent screens arerecommended with headwalls to prevent small animals from entering theoutlets.

Side Ditches: Ditches dug to carry the water collected from the outlets away from thepavement. This feature is common to both surface and subsurfacedrainage. The side ditches should have a minimum longitudinal grade of0.005 m/m and an adequate freeboard to be effective.

Storm Drains: In urban locations where ditches cannot be dug on the side of the

highway, storm drains are installed to carry the surface and subsurfacerunoff.

Daylighting: In a daylighted pavement section, the edges of the base and subbaselayers are exposed to allow water trapped in these layers to flow directlyinto the side ditch. Such a design is the conceptual opposite of abathtub section.

Hydraulic Design of Permeable Base Systems

The issues involved in designing the main components of a permeable base system arepresented in the following sections. The main topics of discussion are permeable base design,

separator layer design, and edgedrain design. The FHWA microcomputer program DRIP (9)can perform the hydraulic design of these components rapidly and accurately. This program isavailable with the software accompanying the Design Guide. The National Highway Institutetraining course 131026, Pavement Subsurface Drainage Design, (10) provides a summarydescription of the program, along with an example design. The DRIP 2.0 Users Guide ispresented in Appendix TT.

Hydraulic Design of Permeable Bases

The recommended approach for performing hydraulic design of permeable bases is the time-to-drain procedure, which is based on the following assumptions:

Water infiltrates the pavement until the permeable base is saturated. Excess runoff will not enter the pavement section after it is saturated. After the rainfall event ceases, water is drained to the side ditches or storm drains

through edgedrains or by daylighting.

The main parameter of interest in the time-to-drain procedure is the time required to drain thepermeable base to a pre-established moisture level. The design standard based on thisparameter rates the permeable base quality of drainage from Excellent to Poor. Table 3presents guidance for selecting permeable base quality of drainage based on this method.


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Table 3. Permeable base quality of drainage rating based on time taken to drain50 percent of the drainable water.

Quality of Drainage Time to Drain

Excellent 2 hours

Good 1 dayFair 7 days

Poor 1 month

Very Poor Does not drain

For most Interstate highways and freeways, draining 50 percent of the drainable water in 2hours is recommended; however, this is only a guideline. The objective of drainage is toremove all drainable water within a short period of time.

The inputs to the time-to-drain design procedure include basic pavement design and materialproperties such as roadway geometry (cross-slope, longitudinal slope, lane width), thickness of

the permeable base, porosity and effective porosity of permeable base aggregate, andpermeability of the permeable base material. Using these inputs, the time-to-drain parameter iscalculated for a given degree of drainage (U). The final design is then chosen on the basis ofthis information.

The following is a step-by-step procedure for completing the time-to-drain design.

1. Assume a desired degree of drainage (U). For typical highway situations, U = 0.5.2. Select a value for the permeable base thickness (H).3. Determine the coefficient of permeability (k) of the proposed base material through

laboratory testing. Permeable bases should typically have k values of 1000 ft/day orgreater.

4. Calculate the resultant length (LR) and resultant slope (SR) from known roadway longitudinalgrade (S), cross-slope (Sx), and permeable base width (W).

5. Calculate the porosity (N) and the effective porosity (Ne) of the base material from known

6.values of bulk specific gravity (Gsb), unit weight (d), and water loss coefficient (WL).

=

sb

d

GN

*81.91

Ne = N x WL

Typical ranges of bulk specific gravity and unit weight for permeable base material are 2.65to 2.70 and 15.5 to 19.0 kN/m3, respectively. The water loss coefficient for a permeable

2/12

1

+=

x

RS

SWL

( ) 2/122 SSS xR += (2)

(3)

(4)

(1)


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SS-8

base is a function of the type and amount of fines present in the base and can bedetermined from table 4.

Table 4. Water loss values as a percentage of total water (4).

Type and Amount of Fines

Filler Silt Clay

2.5% 5% 10% 2.5% 5% 10% 2.5% 5% 10%

MaterialType Gravel 70 60 40 60 40 20 40 30 10

Sand 57 50 35 50 35 15 25 18 8

Notes:Fines are defined as material passing the No. 200 sieve.For gravel with 0 percent fines, water loss is equal to 80 percent.For sand with 0 percent fines, water loss is equal to 65 percent.

6. Use either equation 5 or DRIP (9) to determine the time required to remove 50 percent ofthe drainable water from the saturated permeable base (U = 0.5). Compare this value to thetarget time-to-drain. If the design is unsatisfactory, continue iterations by changing theinputs until the desired solution is obtained.

)(2

2

50HLSk

LNt

RR

Re

+=

Equation 5 represents a single point calculation of the time-to-drain parameter at a degree ofdrainage of 0.5. DRIP can be used to compute the time required to drain to any desired degree

of drainage or saturation (S). When DRIP is used to compute the time to drain, time-historyplots from an initial fully saturated state to a completely drained state can be obtained.Figure 3 is a sample plot of time to drain against the percent drained.

Time to Drain, hours

Percent Drained

0 1 3 5 7100

20

0

40

60

80

H = 0.15 m

N = 0.3Ne = 0.25

LR = 7.6 m

SR = 0.02 m/m

k = 305 m/day

Figure 3. Percent drained versus time.

(5)


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SS-9

Sensitivity of the Time-to-Drain Procedure and Permeable Base DesignRecommendations. Of all the inputs that go into the calculations, permeability has the greatestinfluence and permeable base thickness the least influence on the time-to-drain parameter. Thetime required to drain a permeable base decreases exponentially with an increase inpermeability. Therefore, to reduce the time to drain cost-effectively, it is recommended that thepermeability be increased by a reduction in fines (a minimum of 1000 ft/day is required for

permeable bases). However, care must be taken to maintain adequate stability in thepermeable base while effecting a reduction in fines. To guarantee reasonable stability, aminimum coefficient of uniformity value, CU, of 3.5 is required for an untreated permeable base.If this cannot be achieved, the base should be treated with either asphalt or portland cement.

Since the thickness does not have a significant effect on the time-to-drain parameter, a value 4inches is recommended for permeable bases. This thickness should provide an adequatehydraulic conduit and lend itself to compaction without segregation.

Separator Layer Design

The issues involved in designing the two types of separator layersdense-aggregate and

geotextilewill be discussed in this section. If a combination of these layers is to be used dueto adverse site conditions (see table 3 to determine when this might be necessary), it isrecommended that the geotextile be placed on top of the separator layer.

Design of Aggregate Separator Layer. The aggregate separator layer must satisfy theuniformity and separation requirements at both the separator layer/subgrade interface and theseparator layer/permeable base interface. The separator layer should serve as an impermeablebarrier to prevent the water in the permeable base from entering the subgrade (permeability lessthan 15 ft/day is desired). The aggregate separator layer design is a three-step process.

Step 1: Check for Aggregate Separator Layer/Subgrade Interface Requirements

The gradation of the aggregate separator layer must meet the requirements for the aggregateseparator layer/subgrade interface listed below:

Separation requirement: D15 (Separator Layer) < 5 D85 (Subgrade)Uniformity requirement: D50 (Separator Layer) < 25 D50 (Subgrade)

DX represents the particle size that x percent of the material is small than by weight.

Theoretically, a spherical particle will be retained until the diameter of the retaining spheres is6.46 times greater than the sphere to be retained. This relationship is shown in figure 4. Bylimiting the D15 size of the aggregate separator layer to less than 5 times the D85 size of thesubgrade, the larger soil particles of the subgrade will be retained, allowing the soil bridging

action to start. By limiting the D50 size of the aggregate separator layer to less than 25 times theD50 size of the subgrade, the gradation curves will be kept in balance.

Step 2: Check for Aggregate Separator Layer/Permeable Base Interface Requirements

Similar requirements must be applied to the permeable base/aggregate separator layerinterface, as listed below:

Separation requirement: D15 (Permeable Base) < 5 D85 (Separator Layer)Uniformity requirement: D50 (Permeable Base) < 25 D50 (Separator Layer)


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D = 6.46 DS

DS

D

Figure 4. Retention of spheres relationship (4).

Step 3: Check for Additional Requirements

The following additional requirements are necessary to ensure that the dense-graded aggregateseparator layer does not have too many fines and is well-graded:

Maximum percentage of material passing the No. 200 sieve should not exceed 12percent.

Coefficient of uniformity should be greater than 20, preferably greater than 40.

The first criterion limits the amount of fines in the aggregate separator layer, and the secondprovides guidance for developing a well-graded aggregate base.

The results of these checks are typically plotted on a gradation chart to develop a designenvelope through which the gradation of the aggregate separator layer must pass. A sampleplot of an aggregate separator layer gradation that satisfies the design checks is shown in figure5. Also plotted on the figure is the design envelope developed from the criteria discussed abovefor sample permeable base and subgrade gradations.

In addition, some States prime the dense-graded separator layer to reduce erosion of fines at itssurface.

Design of Geotextile Separator Layers. The discussion in this section summarizes basicgeotextile design. Detailed guidance is provided in Geosynthetic Design and ConstructionGuidelines(11).

The parameters used to specify the appropriate geotextile to be used as a separator layer arethe apparent opening size (AOS) and the gradient ratio (GR). The ability of a geotextile to retainsoil particles is directly related to its AOS value which is the apparent largest hole in the


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Percent Passing

Grain Size - mm

Selected

separator layer

gradation

20

40

60

80

100

0

5 100.50.10.050.01 50 100

Design

envelope

1

SubgradePermeable

base

Figure 5. Plot of design envelope superimposed on base, subgrade, and the dense-graded aggregate gradations (10).

geotextile. The AOS value is equal to the size of the largest particle that can effectively passthrough the geotextile in a dry sieving test (11). The gradient ratio is a measure of thesoil/geotextile clogging potential. ASTM D-4751, Determining Apparent Opening Size of aGeotextile, and ASTM D-5101, Measuring the Soil-Geotextile System Clogging Potential by theGradient Ratio, are standard tests normally employed to measure AOS and GR, respectively.

The important design criteria to be considered in specifying the properties of geotextile as aseparator layer are divided into four categories, namely:

Soil retention. Permeability. Clogging. Survivability and endurance.

The design guidelines for the soil retention, permeability, and clogging criteria are summarizedin the flowchart in figure 6. In addition to these criteria, to ensure that the geotextile will survive

the construction process, certain strength and endurance properties are required. AASHTO-AGC-ARTBA Task Force No. 25provides general guidelines for the selection of the minimumphysical requirements of the geotextile (12, 13). This ensures that the geotextile has adequatestrength and durability to survive both construction and long-term use. A relatively heavy(weight-to-area ratio of 0.03 kg/m2), non-woven geotextile is recommended for separator layerapplications.

Edgedrain Design

The hydraulic design of edgedrains is basically a four-step process. Each step is explainedbelow.


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Soil Retention Criteria

Less than 50% of subgrade material

passes 0.06-mm sieve

Greater than 50% of subgrade material

passes 0.06-mm sieve

Steady State Flow Steady State Flow Non-Steady State Flow

O95 < BD85Subgrade

soil can

move

beneath

geotextile

Subgrade

soil cannot

move

beneath

geotextile

Woven

geotextile

Non-woven

geotextileO50 < 0.5D85

O95 < D15 O50 < 0.5D85

B = 1 Cu < 2 or > 8

B = 0.5 2 < Cu < 8

B = 8/Cu 4< Cu < 8O95 < D85 O95 < 1.8D85

O95 < 0.3 mm

Non-Steady State Flow

CloggingCriteria

Select geotextile meeting

soil retention,

permeability and less

critical clogging criteria

gradient ratio < 3

Use material with maximum

opening size from the soil

retention criteria.

Woven fabrics:

percent open area > 4%

Non-woven fabrics:

porosity > 30%

Additional criteria:

O95 > 3 D15O15 > 3 D15

Critical/Severe Less

Critical/Severe

Permeability Criteria

Critical/Severe Less Critical/Severe

kgeotextile > 10 ksoil kgeotextile > ksoil

Symbol representations:k = permeability

Ox = opening size in geotextile for which x percent of particles are smaller (mm)

AOS = O95.

Dx = soil particle size for which x percent are smaller (mm).

Figure 6. Flowchart summarizing the soil retention, permeability, and clogging criteriafor selecting the properties of geotextile (10).

Step 1. Determine pavement discharge rate, qd. This is the design flow for calculating thepipe capacity and outlet spacing. It can be determined using any of the following threeapproaches:

(1) Pavement infiltration discharge rate(2) Permeable base discharge rate(3) Time to drain discharge rate.

However, for pavements with permeable bases the time to drain discharge rate calculation isappropriate to ensure consistency with the recommended permeable base design methodology(time to drain). The equation to estimate the design discharge is as follows:

D

edt

UWHNq1

24= (6)


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SS-13

where:qd = Design pavement discharge rate, ft

3/day/m.W = Width of permeable base, m.H = Base thickness, m.Ne = Effective porosity.U = Percent drained in decimal (50 percent is used most often).

td = Time to drain, hr.

For pavements with nonerodible bases, discharge can be computed as follows:

Wqq id =

where:qi = Design pavement infiltration rate, m

3/day/m2W = Width of permeable base, m

Pavement infiltration, qi, can be estimated using the crack infiltration equation:

where:Ic = Crack infiltration rate, m

3/day/m2 of pavement surface, typically0.223 m3/day/m2

N = Number of lanesW = Pavement width, mCs = Transverse crack spacing, m

= Joint spacing for JCP; 10 to 30 m for CRC; 5 to 20 m for ACpavements

Step 2. Determine edgedrain flow capacity, Q

For pipe edgedrains, the flow capacity of circular pipes can be determined from Manningsequation:

2

1

3

8310*2693.0SD

nQ

=

where:

Q = Pipe capacity, m3

/dayn = Mannings roughness coefficient

= 0.012 for smooth pipes and 0.024 for corrugated pipes (4)D = Pipe diameter, mmS = Longitudinal slope, m/m

For prefabricated geocomposite edgedrains, the flow capacity is given by the following equation:

2/1

21

+=L

DDSCDQ

(7)

(8)

(9)

(10)

+

+=

s

ciCW

NIq

11


16/27

SS-14

where:Q = Geocomposite edgedrain capacity, m3/day/mC = Manufacturer supplied PGED flow factor, m3 /day/mm. (typical

range = 0.5 to 2.5)D = Averaged depth of flow = (D1 + D2)/2, mmD1 = Depth of flow zone, m

D2 = Depth of outlet (outlet pipe diameter), mS = Longitudinal slope, m/mL = Outlet spacing, m

Figure 7 presents a schematic diagram of flow in a PGED, illustrating the various inputs to theequation above.

D1D

D2

Trench bottom

Freeboard

Flowzone

Outlet pipe

Down stream end ofgeocomposite (outlet)

X

Outlet spacing

A

A

Upstream end of geocomposite

Assumed water surfaced

Possible water surface

Sand backfill

Aggregate BaseFreeboard

Flowzone (D )1

Depth of flow(varies)

Depth of water atoutlet pipe (D )2

Outlet pipe

Water level(at outlet)

Average depth of flow =D + D

2

1 2

ShoulderPavement

100 mm

Geocomposite edgedrain

Figure 7. Schematic diagram for computing flow of PGEDs (9).

Step 3. Determine outlet spacing, L

Once the pavement discharge rate (qd) and the edgedrain flow capacity (Q) have been

determined, the outlet spacing (L) can be determined from the following equation:

dq

QL

Step 4. Determine the trench width

The edgedrain trench must be wide enough to transmit the water discharging from thepavement structure without interrupting the flow. In general, if a permeable backfill material isused, the width needed for the installation of the pipe drain is more than adequate to meet the

(11)


17/27

SS-15

hydraulic requirement. The following equation can be used to ensure that the trench width isadequate to meet the hydraulic requirement:

000,1*k

qW dT =

where:WT = Required minimum trench width, mmqd = Pavement discharge rate, m

3/day/mk = Permeability of the backfill material, m/day

Edgedrain Maintenance Requirements. Pipes with a minimum diameter of 4 inches arerequired for longitudinal pipe edgedrains and outlets (6 inch pipes are commonly used inpractice). This allows easy access of monitoring and maintenance equipment to the pipeinteriors. Further, a maximum outlet spacing of 75 m is recommended. Dual outlets withheadwalls are also recommended. The details of the recommended pipe edgedrain layout in apavement are presented in figure 8.

The maintenance requirements for pipe diameter and outlet spacing often satisfy the hydraulicdesign requirements.

Figure 8. Sketch of pipe layout, dual inlet/outlet systems, and headwalls (14).

Construction of Permeable Base Systems

Permeable Base Construction Considerations

There is no specific procedure for constructing permeable bases. The standard procedures andspecifications for placing AC or portland cement stabilized materials on roadways will beadequate, and most SHAs use procedures that seem best for their environs. The majorobjective is to make sure that the material is placed in a manner that provides a uniform, stable,non-erodible, and non-stripping supporting layer. The presence of qualified personnel aware ofdrainable base construction is recommended.

(12)


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SS-16

Subgrade Support. Good subgrade support is vital in permeable base construction. Aminimum resilient modulus of 63 MPa is recommended. A strong subgrade provides a goodconstruction platform and enhances the performance of the separator layer. A strong subgrademinimizes intermixing at the subgrade/separator layer and separator layer/permeable baseinterfaces, regardless of the type of separator being used (15). This will have a direct impact onpavement performance. However, when thick granular embankment layers are used (for

example in deep frost areas), the subgrade strength requirement can be relaxed.

Hauling. Good practice dictates that traffic be minimized and restricted to low speeds withminimal turning. Permeable bases should not be used as a haul road even for light constructiontraffic. Equipment that could cause rutting, dirty equipment, and equipment transporting finesshould not be allowed to traverse over the permeable base (16). If concrete trucks are allowedon the base, a stabilized permeable base should be used (4).

Control Strip. It is recommended that a control strip be constructed prior to constructing apermeable base so that the combination of aggregate materials and construction practices canbe tested and, if necessary, adjusted to produce a stable permeable base. The test sectionshould be constructed using the same aggregate materials and compaction practices that will

be used on the project. A minimum length of 150 m is recommended for the test section, andthe test section should become part of the finished roadway (10, 14).

In-Place Permeability. The permeability of the permeable base can be reduced to anunacceptable level by overcompaction or contamination with fines. The in-place permeablebase should accept the inflow of water without ponding or flowing across the surface. In-placepermeability tests for permeable bases are difficult to run but may be conducted to obtainestimates.

Environmental Conditions. Permeable base layers should be placed when the airtemperature is above 5 oC. Areas of completed permeable bases that are damaged by freezing,rainfall, or other weather conditions, or are contaminated from sediments, dust, dirt, or foreign

material, should be corrected to meet specified requirements.

Special Considerations for Untreated Permeable Base Construction

The permeable base materials must be placed in a manner to prevent segregation andto obtain a layer of uniform thickness. Extra care should also be taken while stockpilingand handling the materials. An asphalt paving machine can be used to place thepermeable base. A hopper-type base spreader box may be used if it can be operated toobtain specified thicknesses.

The purpose of compacting a permeable base is to seat the aggregate. Most SHAsspecify one to three passes of a 4.5- to 9-metric ton steel-wheeled roller. Over-rollingcan cause degradation of the material and a subsequent loss of permeability. Vibratory

rollers should be used with care to compact unstabilized permeable bases, because theycan cause degradation, over-densification, and a subsequent loss of permeability (14).They can also cause liquefaction of wet separator and subgrade layers, causingcontamination and intermixing (17).

Special Considerations for the Construction of PATB

The PATB material should be spread at a temperature between 90 to 120 oC asmeasured in the hopper of the paving machine. Compaction should begin when the


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temperature of the PATB has cooled to 65 oC and should be completed before thetemperature falls below38 oC (14).

One to three passes of a 4.5- to 9-metric ton steel-wheeled static roller should beadequate. Vibratory rollers should not be used.

Construction traffic should not be allowed on the completed base during the next 24hours.

To ensure initial mix stability, experienced State agencies use: Crushed aggregates. Stiffer asphalt cement grades. Temperature control prior to rolling. Consider stripping susceptibility of PATB.

Special Considerations for the Construction of PCTB

The PCTB should be placed on the grade using a spreading machine or a subgradeplaner. A paver should follow the spreader machine. A subgrade planer attached withvibrating pans can be used for compacting the mix. Vibratory plates and screeds canalso be used to place and compact. Tandem steel-wheeled rollers should not be usedfor compacting PCTB.

It is difficult for a standard concrete paver to place PCTB with high cement contents (>167 kg/m3) and low water-to-cement ratios (< 0.4). The PCC requirement should beoptimized to achieve the required strength, durability, and permeability. The engineershould determine the exact amount of portland cement required to ensure that allaggregates are well coated. A water-to-cement ratio of 0.45 should be used to increaseworkability.

There is no consensus on the most suitable method for curing PCTB. Some Statehighway agencies cover the permeable base with polyethylene sheeting for 3 to 5 daysand apply a fine water mist cure several times on the day after the base is placed. Other

agencies do not cure their PCTB at all because test data show that there is no significantdifference in strength of cured and uncured PCTB. It is therefore recommended that atest strip of the PCTB be constructed and tested with the different curing methodsavailable. The best method based on strength and performance should then be selected.

Construction of Aggregate Separator Layers

Generally, a dense-graded aggregate material is used as a separator layer. It should be aminimum of 4 inches thick. The aggregate separator layer is as important as the permeablebase and subgrade in developing a strong pavement section. This layer is necessary to providea stable platform for placing the permeable base and pavement surface layer, and it should beconstructed using the normal construction techniques for dense-graded aggregate bases and

subbases. On poor soils, up to 12 inches of gravel over a geotextile may be needed (17).

The aggregate separator layer should not experience any rutting or movement during the pavingoperation. Since most SHAs use a dense-graded aggregate for the separator layer, thismaterial should be strong enough to support the paving operation. The material should becompacted until a density of 95 percent of the maximum density is reached, as determined byAASHTO T 180-90, Moisture Density Relationship Using a 4.54 kg Rammer and a 457 mmDrop, Method D.


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Construction of Pipe Longitudinal Edgedrains

Proper construction of edgedrain systems is the key to providing good drainage. Unfortunately,edgedrains are often rendered unusable immediately after installation from a variety ofconstruction-related accidents. Therefore, it is recommended that all the edgedrains be videoinspected soon after construction. Details on video inspection are provided later in this

appendix.

Trenching. The trench should be cut deep enough to place the top of the drainage pipe aminimum of 2 inches below the bottom of the permeable base. A minimum 2-in layer of beddingmaterial is also recommended beneath the drainage pipe. These requirements place thebottom of the trench 8 inches below the bottom of the permeable base for 4-in pipes. Thetrench should be cut at a constant depth so that the bottom of the trench follows the pavementgrade. To obtain proper line and grade, the bottom of the trench should be shaped (or grooved)to cradle the lower one-third of the pipe. The bedding groove helps to hold the pipe in placeduring installation. For the grooving to be effective, the shape of the groove must closely matchthat of the pipe being installed; an oversized bedding groove can do more harm than good (18).

Placement of Geotextile. The edgedrain trench should be lined with a geotextile to preventmigration of fines from the surrounding soil into the drainage trench. However, the top of thetrench adjacent to the permeable base should be left open to allow a direct path for water intothe drainage pipe, as shown in figure 3.

Placement of Drainage Pipes and Backfilling. If a layer of bedding material will be placedprior to placing the drainage pipes, the grooving of the trench bottom has to be done afterplacing the bedding material. When placing CPE pipes, extra care is also required to preventoverstretching during installation. The typical limit for tolerable longitudinal elongation of CPEpipes is 5 percent.

The backfill material should be placed using chutes or other means to avoid dumping the

material onto the pipe from the top of the trench (18). To prevent displacement of drainagepipes during compaction, the backfill material should not be compacted until the trench isbackfilled above the level of the top of the pipes. To avoid damage to the pipes duringcompaction, a minimum of 6-in of cover is recommended before compaction. Achievingadequate consolidation in a narrow trench can be difficult. Inadequate compaction can lead tosettlement, which in turn will result in shoulder distresses. A minimum density of 95 percentStandard Proctor (AASHTO T-99) is recommended. Satisfactory compaction can be achievedby running two passes (two lifts, one pass per lift) with a high-energy vibratory wheel (19).

Automated equipment has been developed that can be used to install flexible pipes. Figure 9shows an example of the equipment used in Illinois that can install pipe drains at a rate of about5 km per day. This equipment has the following features (18):

A tractor at the front end to provide locomotion. A chain-type trencher mounted on a floating boom. The depth of trench can be

controlled by a mechanism activated by a remote laser benchmark, or the trench can bemade a fixed depth below grade.

Either a spiral screw or conveyor belt mounted below the trencher to deflect theexcavated spoil falling from the scoops to each side of the trench.

A boot functioning as a trench shield to support the trench wall and exclude crumbswhile the pipe is feeding through.


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Corrugated plastic tubing

Hopper

Compactor

Gate

CompactorGroover

Digging chain

Boot

Figure 9. Automated equipment to install pipe edgedrains (18).

A groover located on the leading edge of the boot to form a semicircular bedding groovein the bottom of the trench. A hopper that holds and distributes the backfill material. Asmall vibrating compactor located within the hopper compacts the backfill.

A second compactor on the trailing edge of the boom compacts the trench top.

Construction of Prefabricated Geocomposite Edgedrains

Geocomposite edgedrains are typically used in retrofit projects. The installation of these types

of edgedrains involves trenching, installation of the panel drain, connecting drainage outlets,and backfilling. The recommended installation detail for geocomposite edgedrains is shown infigure 10.

100 to 150 mm

PGED

Sand backfill

PavementShoulderCL

Nonerodible base

13 to 25 mm

300 to

450 mm

Figure 10. Recommended installation for geocomposite edgedrains (20).


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PGEDs typically have an inside cross-sectional thickness of 0.5 to 1 inch and a depth of 12 to18 inches. The trench should be cut 4 to 6 inch wide and deep enough to place the top of thePGED 2 inches above the bottom of the pavement surface layer. The PGED should be placedon the shoulder side of the trench, and the trench should be backfilled with coarse sand toensure intimate contact between the geotextile and the material being drained. Achieving thiscontact is very important to prevent loss of fines through the geotextile. The PGED should be

connected to the outlet pipes prior to backfilling.

For geocomposite edgedrains, excessive compaction can cause crushing and buckling of theedgedrain panels. The recommended procedure is to backfill using coarse sand and compactby flushing with water (20). The cuttings from the drainage trench are not a suitable backfillmaterial when installing a geocomposite edgedrain. If the panel design is not symmetrical aboutthe vertical axis, the panel should be installed with the rigid or semi-rigid back facing the sandbackfill (21).

Maintenance of Subsurface Drainage Systems

Of equal importance to providing subsurface drainage systems is the maintenance of thesystems. Although very little can be done by way of maintenance to separator layers andpermeable bases once they are constructed, post-construction maintenance is of paramountimportance for proper functioning of the pipe drains, outlets, headwalls, and roadside ditches.Maintenance of the exposed periphery of daylighted bases is also critical.

Importance of Proper Maintenance

An improperly maintained system can clog and cause the pavement structure to becomeflooded with excess watera condition that is usually worse than if no system was provided atall. Some of the common problems that occur as the result of improper maintenance ofsubsurface drainage systems with edgedrains are discussed below.

For permeable bases with longitudinal edgedrains:

Crushed or punctured outlets that are left unattended for long periods of time. Outlet drains that are clogged with debris, mice nests, mowing clippings, vegetation, and

sediment.

Edgedrains (both pipe drains and fin drains) that are filled with sediment, especially atsags and slopes of less than 0.01 m/m.

Missing rodent screens at the outlets. Missing outlet markers. Erosion around outlet headwalls and damaged headwalls. Shallow ditches that have inadequate slopes and that are clogged with vegetation.

For daylighted permeable bases:

Excessive vegetative growth over the daylighted portion. Deposition of roadside debris. Silting of the daylighted openings.


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Typical Inspection Techniques

Table 5 presents some typical inspection activities for daylighted and edgedrained permeablebase systems, along with the frequencies at which they should be performed. To maintain theeffectiveness of a subsurface drainage system with a daylighted permeable base, annual visualinspections are recommended. However, internal and external visual inspection of longitudinal

pipes, outlet pipes, and ditches are essential for a system with edgedrains. Several inspectiontechniques and types of equipment can be used effectively for internal and external evaluationof edgedrain systems (2226). Notable among these is the video inspection of edgedrain andoutlet systems.

Table 5. Summary of various inspection activities and recommended frequencies (10).

RecommendedInspection

ActivityPurpose Frequency of

Inspection

DaylightedPermeableBase

Visual inspection

Check vegetative growth,silting

Yearly

Site information Inventory data collection Once at the start ofproject inspection

(update asrequired)

Roadwaycondition

Detect moisture-relateddistress

Once every twoyears

Visual externalinspection

Check slopes, outlet markers,vegetation, etc.

One to two times ayear

Video monitoring Check internal pipe condition One to two times ayear

PermeableBase withEdgedrains

Informationlogging

Record data and assessmaintenance needs As needed

Video Inspection of Edgedrains. Equipment that is typically required for the video inspectionprocess includes a closed-circuit video camera, portable generator, weed eater, metal detector,and miscellaneous tools (e.g., shovels, crow bars, tape). Where there are questions aboutwhether outlets can effectively drain into shallow ditches or medians, equipment for measuringditch depths and the vertical drop to outlets is necessary. A detailed description of theequipment used in the recent FHWA study to inspect pipe edgedrains is presented in table 6.Figure 11 illustrates the camera system used for pipe inspections. For PGEDs, a rigidborescope camera system is used to check for clogging.

Typical Maintenance Activities

Maintenance activities can be grouped under two categories: routine maintenance and need-based maintenance. Routine maintenance activities need to be performed at least once a year,whereas need-based maintenance activities are performed only as the need arises.

Daylighted Bases. Typical routine maintenance activities on daylighted bases include weedingand manual removal of debris. Moderate flushing with a water jet can be performed to removesilt deposits in the daylighted openings. Care should be taken not to use a high-pressure jet, asit could result in permeable base damage. After the base is free of debris, a visual inspection


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Table 6. Equipment description for FHWA video inspection study (26).

Camera - The camera is a high-resolution, high-sensitivity, waterproof color videocamera engineered to inspect pipes 3 to 6-in in diameter. The flexible probe attached tothe lighthead and camera has a physical size of 2.75 in and is capable of negotiating 4 x

4 in plastic tees. The lighthead incorporates six high-intensity lights. This lighting providesthe ability to obtain a true color picture of the entire surface periphery of a pipe. Thecamera includes a detachable hard plastic ball that centers the camera during pipeinspections.

Camera Control Unit The portable color control unit includes a built-in 8-in colormonitor and controls including remote iris, focus, video input/output, audio in with built-inspeaker, and light level intensity control. Two VCR input/output jacks are provided forvideo recording as well as tape playback verification through the built-in monitor.

Metal Coiler and Push Rod With Counter The portable coiler contains 150 m ofintegrated semi-rigid push rod, gold and rhodium slip rings, electro-mechanical cablecounter, and electrical cable. The integrated push rod/electrical cable consists of aspecial epoxy glass reinforced rod with polypropylene sheathing material, which will allowfor lengthy inspections due to the semi-rigid nature of this system.

Video Cassette Recorder - The video cassette recorder is a high-quality four-headindustrial grade VHS recorder with audio dubbing, still frame, and slow speedcapabilities.

Generator - A compact portable generator capable of providing 650 watts at 115 V topower the inspection equipment.

Molded Transportation Case - A molded transportation case, specifically built for airtransportation, encases the control unit, camera, and videocassette recorder.

Color Video Printer - A video printer is incorporated into the system to allow thetechnician to obtain color prints of pipe anomalies or areas of interest.

Figure 10. Camera system used for pipe inspections.


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should be conducted right after a rainfall event or by dumping water on the pavement from awater truck to assess the impact of the maintenance activities on base drainability.

Permeable Bases with Edgedrains. Common routine maintenance activities include thereplacement of damaged drains and outlets, replacement of headwalls, and removal ofvegetation from around the drain outlets. Another key parameter to effective drainage system

maintenance is the clear identification of outlet locations. If maintenance crews cannot locatethe outlets, they cannot clean and maintain them. In addition to these tasks, it is alsorecommended that the drainage systems be flushed and rodded at least once every 2 years asa preventive measure.

If clogged drains pose a severe problem, flushing using high-pressure water is recommended(26, 27). A high-pressure rodding system called a jetter has been used successfully in manyStates for clearing clogged edgedrains and outlets.


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REFERENCES

1. McAdam, J.L. Report to the London Board of Agriculture, London, England, 1820.2. Hindermann, W. L. The Swing to Full-Depth. Information Series No. 146. Lexington, KY: The

Asphalt Institute, 1968.3. Hagen, M. G., and G. R. Cochran. Comparison of Pavement Drainage Systems. Report

Number MN/RD-95/28. Maplewood, MN: Minnesota Department of Transportation, 1995.4. Federal Highway Administration. Drainable Pavement Systems Participant Notebook.

(Demonstration Project 87). Publication No. FHWA-SA-92-008. Office of TechnologyApplications and Office of Engineering, 1994.

5. Highway Research Board. Final Report on Road Test OneMD, Special Report 4.Washington, DC: Highway Research Board, 1952.

6. Highway Research Board. The WASHO Road Test. Special Reports 18 and 22.Washington, DC: Highway Research Board, 1955.

7. Highway Research Board. The AASHO Road TestReport 5, Pavement Research. SpecialReport 61E. Washington, DC: Highway Research Board, 1962.

8. Carpenter, S. H., M. I. Darter, and B. J. Dempsey. Evaluation of Pavement Systems forMoisture-Accelerated Distress, Transportation Research Board. Record No. 705.Washington, DC: Transportation Research Board, 1979.

9. Wyatt, T., W. Barker, and J. Hall. Drainage Requirements in Pavements User Manual.Report No. FHWA-SA-96-070. Washington, DC: Federal Highway Administration, 1998.

10. ERES Consultants, Inc. Pavement Subsurface Drainage DesignReference Manual.Washington DC: Federal Highway Administration, 1999.

11. Holtz, R.D., B.R. Christopher, and R.R. Berg. Geosynthetic Design and ConstructionGuidelines, Participants Notebook. Publication No. FHWA-HI-95-038, Washington, DC,1998.

12. AASHTO. Task Force 25 ReportGuide Specifications and Test Procedures forGeotextiles. Washington, DC: AASHTO, 1990.

13. AASHTO. Standard Specifications for GeotextilesM 288. Standard Specifications forTransportation Materials and Methods of Sampling and Testing, 18th ed. Washington, DC:

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Graded Drainage Layers. Project C960014. Illinois Transportation Research Center, IllinoisDepartment of Transportation, 1998.

15. Christopher, B. R., and V. C. McGuffey. NCHRP Synthesis 239. Pavement SubsurfaceDrainage Systems. Washington, DC: TRB, National Research Council, 1997.

16. Baumgardner, R. H. Overview of Pavement Drainage Systems for Concrete Pavements.Proceedings of the International Symposium on Subdrainage in Roadway Pavements andSubdrains. Grenada, Spain, 1998.

17. Laguros J., G. Miller. Stabilization of Existing Subgrades During Interstate PavementRehabilitation to Improve Constructability. NCHRP Synthesis 247. Washington, DC:Transportation Research Board, 1998.

18. Chambers, R.E., T.J. McGrath, and F.J. Heger. Plastic Pipe for Subsurface Drainage ofTransportation Facilities. NCHRP Report 225. Washington, DC: Transportation ResearchBoard, 1980.

19. Ford, G.R., and B.E. Eliason. Comparison of Compaction Methods in Narrow SubsurfaceDrainage Trenches. Transportation Research Record 1425. Washington, DC:Transportation Research Board, 1993.

20. Koerner, R.M., G.R. Koerner, A.K. Fahim, and R.F. Wilson-Fahmy, Long-Term Performanceof Geosynthetics in Drainage Applications. NCHRP Report 367. Washington, DC:Transportation Research Board, 1994.


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21. Fleckenstein, L.J., D.L. Allen, and J.A. Harison. Evaluation of Pavement Edge Drains andthe Effect on Pavement Performance. Report KTC-94-20. Frankfort, KY: KentuckyTransportation Cabinet, 1994.

22. Steffes, R.F., V.J. Marks, and K.L. Dirks. Video Evaluation of Highway Drainage Systems,Transportation Research Record 1329. Washington, DC: Transportation Research Board,1991.

23. Ahmed, Z. and T.D. White. Methodology for Inspection of Collector Systems,Transportation Research Record 1425. Washington, DC: Transportation Research Board,1993.

24. Sawyer, S., and C.J. Hayes. Establishment of Underdrain Maintenance Procedures,Oklahoma City: Oklahoma Department of Transportation, 1995.

25. Hassan, F.H., T.D. White, R. McDaneil, and D. Andrewski. Indiana Subdrainage Experienceand Application, Transportation Research Record 1519. Washington, DC: TransportationResearch Board, 1996.

26. Daleiden, J.F. Video Inspection of Highway Edgedrain Systems. Report FHWA-SA-98-044.Washington, DC: Federal Highway Administration, 1998.

27. Cancela, M.D., J.R. Graciani, J.J. Vaquero. Performance of Concrete Pavement DrainageSystems in Spain, 7th International Symposium on Concrete Roads. Vienna, Austria,

October 3-5, 1994.

guide for me for new and rehabilitated pavement structures

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